The role of heat treatment in light oxidation of fluid milk

Light-oxidized flavor (LOF) resulting from photooxidation of riboflavin following light exposure is one of the most common off-flavors in fluid milk. The sensory perception of LOF has been studied extensively in high temperature, short time pasteurized (HTST) milk, but few studies have evaluated ultrapasteurized (UP) milk. The objective of this study was to evaluate the role of heat treatment in the development of LOF in UP fluid skim milk. Skim milk was processed by HTST or by direct steam injection (DSI-UP) and subsequently exposed to 2,000-lx light-emitting diode light for various times. Sensory properties were monitored by descriptive analysis and threshold tests, and volatile compounds were evaluated by solid phase microextraction with gas chromatography-mass spectrometry. Dissolved oxygen and riboflavin were determined at each time point using an oxygen meter and ultra-performance liquid chromatography with a fluorescence detector, respectively. The entire experiment was performed in triplicate. Typical cardboard and mushroom flavors (LOF) were detected by trained panelists in HTST milk after 3.5 h of light exposure. In contrast, LOF was not detected by trained panelists in UP milk until 36 h of light exposure. Similarly, the best estimate threshold for LOF from untrained consumers (n = 101) was higher for DSI-UP milk (61.0 h) than for HTST milk (15.2 h). Milks with LOF were characterized by higher relative abundance of the lipid oxidation compounds hexanal and heptanal. Dissolved oxygen (DO) and riboflavin concentrations decreased with increased light exposure time, and the decrease was slower in UP milk compared with HTST milk. Initial DO concentration was investigated as a possible influence in LOF development because DSI-UP milks had lower initial DO concentrations than HTST milks. However, follow-up evaluations of deaerated HTST milks suggested that DO was not a significant factor in LOF development. These results demonstrate that UP milk is less sensitive to LOF than HTST milk, possibly due to sensory masking effects or antioxidant effects of volatile sulfur compounds. An enhanced understanding of light and storage effects on milks will assist with best practices when transporting and displaying fluid milk products for sale.